Scraper blades are used in conveyor belt cleaning mechanisms to remove material that adheres to the belt surface after the material has passed the material discharge point of the conveyor. It is advantageous to provide the scraper blade with a range of motion which allows the scraper blade to follow the obstruction to allow the obstruction to: pass over the blade without significant change in biasing forces and without causing damage to the scraper blade or to the surface; and to also enable the scraper blade to follow any undulations in the surface to be cleaned to provide substantially continuous removal of material from the surface. A scraper blade is provided with a range of motion by providing the scraper blade with one or more pivot points, or one or more pivot axes, about which the scraper blade may pivot. A biasing mechanism is provided that will return the scraper blade to its original scraping orientation if the scraper blade is pivoted away from the surface by engagement with a fixed obstruction in order to maintain relatively continuous scraping engagement with the surface.
Conventional conveyor belt cleaning devices generally include one or more scraper blades disposed in a plane transverse to the conveyor belt surface and are urged toward the conveyor belt so as to cause engagement of a scraping edge of the blade against the conveyor belt surface. Such conveyor belt cleaning devices are often located rearward of the head pulley drum on the return path of the conveyor belt so that sufficient flexibility in the belt is provided to permit transverse movement thereof when protruding objects pass the cleaning device, such as a mechanical splice in the conveyor belt. The cleaning angle of a scraper blade, which is the angle between the surface of the conveyor belt and the face of the scraper blade that faces the oncoming belt, may be an acute angle of less than 90 degrees, a right angle of 90 degrees, or an obtuse angle of greater than 90 degrees. As used hereinafter, “obtuse angle” shall include a right angle of 90 degree.
When a scraper blade is orientated with respect to the conveyor belt surface at an obtuse angle, it is referred to as the “peeling” angle. When a scraper blade is orientated with respect to the conveyor belt surface at an acute angle, it is referred to as the “scraping” angle. It has been found that scraper blades used in connection with conveyor belts conveying certain bulk materials with fines, such as ores, clean the conveyor belt more efficiently when the scraper blades are orientated at a peeling or obtuse angle with respect to the conveyor belt, rather than at a scraping or positive angle.
While an obtuse cleaning angle is preferable, from an efficiency point of view, over an acute cleaning angle, and a resiliently mounted blade is preferable to a rigidly mounted blade, resiliently mounted scraper blades having an obtuse cleaning angle can be subject to destructive and cleaning efficiency reducing vibration at the scraping edge of the scraper blade. When a scraper blade is engaged against the belt surface, a rotational moment is created in the scraper blade by the drag of the belt over the edge of the scraper blade. When pressed against a conveyor belt with a predetermined force to generate a predetermined cleaning pressure, a resiliently mounted scraper blade arm having an acute angle will rotate in a direction away from the belt surface, thus reducing the drag force created by the belt. This results in a comparatively small rotation and consequently only a small change in orientation from the scraper blade's natural orientation, thus producing a relatively stable and constant relationship between the blade and the belt surface with little or no resulting scraper blade vibration.
When a resiliently mounted scraper blade has an obtuse angle and an obtuse scraper blade arm where the primary point of contact is ahead of the behind scraper blade arm, and is applied against the belt with the same predetermined cleaning pressure, it will rotate in a direction toward the belt surface, thereby increasing the drag force. The increasing drag force will cause a reaction, wherein the blade and blade support mechanism to rotate away from the belt surface. Once again, the predetermined cleaning pressure will cause the blade arm to again rotate toward the belt surface, increasing the drag force. This unstable fluctuating relationship between the blade and the belt surface is seen as vibration of the scraping edge of the scraper blade relative to the belt surface, which is undesirable and destructive to the scraper blade and conveyor belt surface.
Another problem that is encountered by both peeling angle and scraping angle conveyor belt cleaners is the tendency of the conveyor belt cleaner or the belt to become damaged when the conveyor belt reverses direction or “rolls back” momentarily. Conveyor belt reversal or roll back happens frequently when the conveyor belt goes through a normal shutdown sequence. The belt thereafter rolls backward from a few inches to a few feet as the belt tension relaxes. This roll back motion tends to catch the scraper blade, forcing the blade to arc and bending the arm on which the scraper blade is mounted, or otherwise causing the scraper blade to swivel out of alignment. Roll back is particularly damaging when the scraper blades do not swivel back into their original cleaning position when the conveyor belt is restarted, or the damage to the scraper blade mounting assembly is so severe that the belt cleaner no longer functions properly. Even worse is the potential for damaging the belt when it resumes forward motion if the scraper blade is locked in a position where only a corner of the scraper blade is in engagement against the belt.
A common situation in which conveyor belt damage occurs frequently is in an inclined conveyor that is shut down with a load on the belt. The load tends to pull the conveyor in a reverse direction, sliding downhill. To prevent the belt from running away downhill with a load on it, a device called a “hold-back” is incorporated either onto the shaft that drives the pulley or within the gearing. The hold-back device does not operate with 100% efficiency, and it is common for the belt to creep backward. A very short distance of creep back or roll back, for example, ⅛ inch of roll back, is sufficient to impose tremendous loads on the scraper blades since the scraper is usually working at an angle to the conveyor belt. Such compression loads will damage the weakest link first, with the weakest link usually being the scraper blades. If the scraper blade is solid and strong enough, it will cause tearing damage to the conveyor belt. Because of this condition, most conveyor belt cleaners are designed to engage an unsupported section of the belt after it has left the pulley. In such arrangements, the belt does not have a solid backing, and the belt itself can lift, thereby relieving the forces that would be imposed on the scraper in response to a reversing condition.
In conventional belt cleaners, a scraper blade is preferably mounted on a support member by a mounting apparatus which provides a controlled range of motion to the scraper blade, and which provides a sufficient range of motion in multiple axes to enable the scraper blade to follow surface undulations and to also enable obstructions to pass over the scraping edge and scraper blade without generating forces applied to the scraper blade that are sufficient in magnitude to cause damage to the blade or the surface to be scraped, or to cause the scraper blade to resonate against the surface.
The provision of a biasing or tensioning force to a scraper blade in conventional belt cleaners to maintain scraping engagement with a surface has been achieved by the use of counter-weights, springs, air bags such as gas-filled shock absorbers, and liquid filled devices, that act in either compression, tension or torsion to pivot or otherwise move the scraper blade into scraping engagement with the surface. These devices provide a return biasing force, which returns the scraper blade to its original scraping orientation, that increases in magnitude as the scraper blade is increasingly displaced from its scraping position or unbiased rest position. However, many of these prior art cleaners can easily get into a resonance state because the resistance friction (which, in the known cleaners, is normally only the friction of the bearings supporting the lever arm) are small compared to the cleaning force.
Accordingly, despite numerous variations within the known art for mounting a scraper blade to a support member, a need still exists for a conveyor belt cleaner having a scraper blade mounting apparatus which will resiliently maintain the scraper blade in biased scraping engagement with the surface to be cleaned with a relatively constant biasing force, while not causing damage to the scraper blade or to the surface being cleaned.